Turkish Airlines Miles&Smiles - Business Companion fare

I found a great C fare BKK-HEL for 1265 euros on flug.sg but when I try to book the price goes up to 4721 euros!
On the TK website the same flight is 1920 euros and I couldn't find any mention of a companion fare deal
Has anyone come across this deal and how/where could I book it?

its a 50% off companion fare that can only be booked in combination with a pax who pays the higher fare (which is twice the price).